Traditionally, incandescent and fluorescent illuminating devices have been used as light sources in some simple devices such as radio. However, significant advances in the light emitting diode (LED) technology have made LEDs attractive for use in more and more fields, such as automobiles and other devices, because of their long operating life, high efficiency, and low profile.
The electrical characteristics of LEDs are such that small changes in the voltage applied to a LED will cause appreciable changes in the current that passes through the LED. The LED luminance is proportional to the LED current and, a small change in the voltage will cause an appreciable change in the LED luminance. Currently, LED drivers use driver circuits that include voltage source outputs with current limiting resistors or linear current regulators. Current limiting resistors will cause power loss, in these ways, current adjustment method with current limiting resistors or linear current regulators is not precise. Driving LEDs with a current larger than a reference current can reduce the LED's life and produce unpredictable light output. As the application of the LED in the automotive industry expands to include high power applications, such as the rear combination lights (Stop/Turn/Tail), the performance of these driver circuits is no longer acceptable in terms of efficiency and regulation. It would be desirable to have a driver circuit for LEDs that would overcome the above disadvantages.
Nowadays, LEDs are used in many new fields because of their environmental durability, long-time durability, high optics efficiency, etc.; consequently, the LED driver and its design also gained more attention. Because of different manufacturing processes employed by different LED manufacturers, the electrical characteristics of the LEDs from one manufacturer may differ greatly from the electrical characteristics of the LEDs from another manufacturer. Furthermore, the electrical characteristics of the LEDs may also vary according to different types of LEDs. Typically, when the forward voltage (VF) of the LED exceeds 3.6 V, even a small increase of the VF will cause the forward current (IF) to substantially increase. The rapid increase of IF will cause the LED to be brighter and hotter, thus accelerating the LED consumption, shortening the LED's useful life, even destroying the LED. Based on the characteristics of the voltage-current change ratio, the LED driver needs a competent design.
There are two types of LED driver chips comply with high-power LED-driver and general LED-driver. Technically, in order to connect more LEDs, the LED driver chips usually use inductor for storage power and PWM pulses which generate internally from the LED chips to drive the LEDs. There are two ways for adjusting a LED's luminance: by regulating the PWM duty ratio and by regulating a LED's bypass current. The method of adjusting LED's luminance by regulating the PWM duty ratio can be used in the devices that comprise CPU. Regulating a LED's bypass current through a potentiometer does not yield to accurate adjustment because under a constant voltage the LED current changes nonlinearly. Consequently, this method causes imprecise light adjustment and light flickers.
Due to LED voltage-current change characteristics, it is recommended to use a constant voltage to drive a LED. Though the Low Drop Out Regulators (LDO) are not precise and not adequate to stabilize currents, the LDOs are commonly used with the LEDs.
Most of the LED chips nowadays use PWM to control the LED luminance. In order to assure people do not see the PWM pulse, the frequency of the PWM pulse must be higher than 100 Hz. But the LED chips are generally designed for adjustment of the LED luminance during the operation. Some chips in the market allow the adjustment of the LED luminance through regulating the PWM pulse, but the adjustment cannot be done manually. However, in practical applications, such as smart lighting, advertisement, automobile, etc., manual luminance adjustment is preferred. Furthermore, it is hoped when the LED's luminance is adjusted by manually, the LED luminance can be adjusted linearly or proximately linearly.
Therefore, it is needed a system that allows easy and linear adjustment of the LED's luminosity without negatively impacting on the LED's performance, and it is to such system and method the present invention is primarily directed.